This invention relates to the field of power switching, and more particularly to a method for automatically controlling switching power losses.
In applications having power semiconductors devices driving reactive loads, such as MOSFETs in voltage converters, a significant source of energy loss and attendant component overheating occurs as these devices change states. Due to the inability of inductances to instantaneously change currents, an On/Off switching of the power semiconductor creates a condition where a high voltage is impressed across the semiconductor switching device simultaneously with a high current being conducted through the switching device. This results in a large power transient that is proportional to the voltage, the current, and the amount of time that the condition exists relative to a cyclic time period. Such power transients reduce system efficiency and can often be damaging to the semiconductor device if not minimized.
Conventional design approaches to alleviating this switching stress on the power semiconductor has been to implement various xe2x80x9csoftxe2x80x9d switching circuits, such as zero voltage switching and zero current switching, where alternative circuit devices are used to sustain inductive currents during the switching transitions until the semiconductor device voltage is minimized. Exemplary techniques can involve the incorporation of one or more reactive xe2x80x9csnubber/slope controlxe2x80x9d resistor-capacitor (RC) circuits to provide a transition charge to a main reactive element during the time that the semiconductor is changing states. During these switching transition times, energy is stored in the snubber circuits and is dissipated as heat during the remainder of a periodic cycle so as to be ready to provide an identical function on a following cycle. This allows the semiconductor device to change states at a minimal current condition, and, thus, with a minimal transient power loss. While this shifts the energy loss away from the semiconductor device to a heartier snubber component, it does not eliminate the loss, so the energy inefficiency still exists.
An alternative approach uses L-C resonant circuits that are excited to a steady-state resonance condition, wherein signal waveform changes are generated by the L-C circuit rather than by the semiconductor device. In the steady state operation of such an application, the semiconductor device can then be switched on exclusively in a low voltage/low power condition. As before, such a configuration requires the use of fixed components that are selected on the basis of a predetermined fixed operating frequency and a maximum transient power control requirement. With such predetermined solutions, system operation and efficiency cannot be optimized for variations in input voltage or output loading.
An electronic apparatus and method for reduce switching power losses in a semiconductor power switching device in voltage converters by calculating a power dissipation signal using a multitude of instantaneous voltage and current samples. This calculated power signal is compared to a reference signal that is derived from previous data samples, and a correction signal is generated to dynamically modify the drive and/or timing parameters of the power switching device. The multitude of voltage and current samples are preferably measured using an analog-to-digital converter (ADC) and accumulatively stored in memory until used by a computing device to calculate the correction signal, which is preferably an adjustment in a Toff time for the switching devices. The constant monitoring of the power dissipation via the ADCs and continuous correction of the power dissipation optimizes the operation of the voltage converter.